This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Treating HIV infection requires combinations of at least 3, and often many more drugs. Several of the drugs used to treat HIV affect how the body eliminates other HIV drugs. Many of these interactions have been studied in adults, but far less is known about their use in children and adolescents. This study will look at the level of HIV drugs over a 12-hour period in children and adolescents who have been prescribed one of three combinations of HIV drugs of interest. This information will help doctors decide how much drug they should prescribe.